Processes for depositing thin films of various materials on substrates are well known and have been found to be very useful. The processes can be broadly classified into two categories: physical vapor deposition and chemical vapor deposition. As used herein, chemical vapor deposition refers to that coating art wherein a plurality of reactive components are introduced into a coating chamber. The components are caused to chemically react with one another, and the products of the reaction form the film that is coated upon the substrate. Chemical vapor deposition processes can be conducted at various pressures and temperatures.
As used herein, the term physical vapor deposition refers to that coating art wherein at least one of the coating components is initially placed into the coating chamber in a non-gaseous form. The non-gaseous coating component is generally called the "source." Sufficient energy is applied to the source material to change it to its vapor state, which vapor subsequently comes to rest in film form on the substrate, perhaps after combining with other components.
There are a number of different physical vapor deposition techniques, which are distinguished in the manner in which the source material is vaporized. One physical vapor deposition technique involves heating the source material in a crucible. The crucible is heated until the contained source material melts and then vaporizes. A related technique involves passing electric current directly through the source material so that the source melts and then vaporizes due to Joule heating. In the latter process, the electrical energy is physically conducted to the source through a metallic conductor, and an arc is not generally created.
Physical vapor deposition also includes ionic bombardment and sputtering deposition techniques. With these techniques, the source material is disposed within the coating chamber as a target and is bombarded with accelerated ions. The bombarding ions impart sufficient energy to the source target material to vaporize it.
Still another type of physical vapor deposition technique, and the one to which this invention pertains, is electric arc vapor deposition. Here, as opposed to the induction Joule heating process described above, an arc is intentionally struck, and the electrical energy contained in the arc is controlled, to vaporize the source material, thus creating a coating plasma. The source material is biased at one electric potential within the coating chamber and acts as one electrode (usually the "cathode") of the electric arc discharge circuit. Another portion of the deposition chamber is biased at a second potential, different from the source potential, and acts as the second electrode (usually the "anode") of the electric arc discharge circuit. An arc-initiating trigger element is positioned proximate to the cathode source and is positively biased with respect to the cathode. The trigger element is momentarily allowed to engage the surface of the cathode material, establishing a current flow path through the trigger and cathode. As the trigger element is removed from engagement with the cathode source, an electrical arc is struck, which is thereafter maintained between the cathode and the anode electrodes of the chamber. In fact, a plurality of such arcs are typically maintained between the two electrodes in an operative electric arc vapor deposition chamber. This electric arc vapor deposition phenomenon is well known, and need not herein be discussed in detail. The electric arc(s) energy is sufficient to vaporize the source material, forming a coating plasma for subsequent deposition onto substrates within the deposition chamber.
One type of coating source material that is often used for the cathode of electric arc vapor deposition machines is titanium (Ti). When a Ti source material is used, a reactive gas such as nitrogen (N) is often introduced into the deposition chamber during the vaporization of the Ti source. The nitrogen gas reacts with the Ti. Thus, the coating plasma within the chamber comprises Ti, N.sub.2, and TiN. TiN forms a gold-colored coating that has been found to be a very durable coating for cutting tools and the like.
The design of the cathode source is critical to an electric arc vapor deposition apparatus. Such cathodes are typically of solid construction, made entirely of the source material to be vaporized and coated upon the substrate. Since the cathode is a consumable/disposable item, its design must be simple and cost effective, maximizing consumable use of the cathode material. Since the cathode is subjected to extremely high temperatures and temperature gradients during a coating run, it must be of a design that will not warp out of shape and be reliably and easily replaceable when spent. Due to the high temperatures involved in the deposition process, the cathode must interface with cooling systems that communicate with the environment external of the deposition chamber. The cathode must cooperatively mate with such cooling systems in a manner so as to reliably maintain the vacuum seal of the deposition chamber and so as to avoid contamination of the coating chamber by the cooling system.
A number of such cathode source material configurations are known in the electric arc vapor deposition art. One such prior art cathode design is illustrated in FIG. 3. Such a cathode is typically constructed of solid titanium, and is turned and threaded from a solid piece of titanium stock material to produce the configuration illustrated in FIG. 3. The cathode is externally threaded adjacent to its rear surface for engaging mating threads of a cathode holder which is in turn supported by the chamber wall of a vapor deposition chamber. A sealing member, generally in the form of an O-ring, peripherally engages the rear annular surface of the cathode to form a seal for the liquid coolant which engages the rear surface of the cathode.
Generally, such prior art cathode construction functions reasonably well. However, it suffers from several drawbacks. For one, the machining process mentioned above is quite expensive and time-consuming. Further, a considerable amount of titanium, which is relatively expensive, is wasted during the turning and threading process of the titanium stock material. Also, due to the relatively large diameter of the threaded portion of such prior art cathode, it is oftentimes difficult to loosen from the cathode holder for replacement of the cathode when the source material thereof needs replenishing. That is, after several "runs" of the electric arc deposition machine, with the cathode having experienced a series of severe thermal expansions and contractions, it has been found that prior art cathode designs such as that of FIG. 3 are difficult to unscrew from their cathode holders. Another drawback of the prior art cathode apparatus is that the cathode holder must be fairly complex, requiring female threads of relatively large radius to mate with the male threads of the cathode.
Still another shortcoming of prior art cathode structures such as that of FIG. 3, is that the liquid coolant which is placed in contact with the rear surface of the cathode during operation of the deposition machine does not typically come into direct cooling contact with the sealing O-ring which is mounted external of the cathode threads. Thus, the O-ring in such cathodes experiences high temperatures and has a tendency to prematurely age and lose its resiliency, a problem that is well known to those skilled in the art of gasket design.
The present invention is directed to the aforementioned problems. The cathode design of this invention requires less machining than its prior art counterparts, can be constructed from less stock source material, and is easier to remove from its cathode holder. A very simple cathode holder will accommodate the cathode of this invention. Also, the present design enables liquid coolant to continually directly contact the cathode holder seal, to preserve the seal life and to prevent the liquid coolant from contaminating the deposition chamber. These and other advantages of the invention will become apparent to those skilled in the art in light of the following description.